This application is related to Japanese Patent Application No. 2001-191488 filed on Jun. 25, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a photodiode, more particularly, to a structure of a photodiode suitable for manufacture of an IrDA communication receiver which operates at high speed and exhibits good noise immunity.
2. Description of Related Art
The IrDA communication (Infrared Data Association communication, or infrared wireless data communication) is a useful data communication method used between computers, computer peripherals, mobile phones, electronic keys and the like. Receivers used with such equipment are required to have quick-response and be immune to electromagnetic noise generated from computers and other equipment.
In a prior-art photodiode for the IrDA communication, as shown in a plan view of FIG. 10 and a sectional view of FIG. 11, an Nxe2x88x92 epitaxial layer 2 is formed on an N+-type substrate 1, and a P-type diffusion layer 3 to be a light-receptive section of the photodiode is formed on the N epitaxial layer 2. An N-type diffusion layer 4 is formed as a channel stopper to surround the P-type diffusion layer 3. The surface of a chip is covered with an oxide film 5. A contact hole is formed in a part of the oxide film, and an Al electrode 6 to be an anode is formed at the contact hole. An Au electrode 9 to be a cathode is formed on a back surface of the chip. Since the IrDA communication is used for receiving data from a distance ranging from several tens of centimeters to several meters, the level of signals varies greatly. In addition to that, the electromagnetic noise from computer equipment often causes mis-operation of the photodiode having the prior-art structure.
In accordance with Japanese Unexamined Patent Publication No. HEI 11(1999)-298033, in a photodiode in which diffusions are distributed on a surface of a chip, a dummy electrode is provided to oppose an electrode connecting the diffusions, and a signal from the distributed photodiode and a signal from the dummy electrode are cancelled by a differential amplifier.
In a photodiode of this structure, as shown in a plan view of FIG. 12 and a sectional view of FIG. 13, a Pxe2x88x92 epitaxial layer 2A is formed on a P+-type substrate 1A. N-type diffusion layers 3A to be light-receptive sections of the photodiode are formed in the form of a plurality of islands on the epitaxial layer. The island-form light-receptive sections are connected by Al wiring lines 6A which are cathodes, the Al wiring lines 6A forming a comb-like configuration. Other Al wiring lines 7 are provided as dummy electrodes between the Al wiring lines 6A in such a manner that the Al wiring lines 7 are opposed to the Al wiring lines 6A. The Al wiring lines 7 as the dummy electrodes are electrically insulated from anodes and cathodes. If noise comes from outside, the cathodes 6A and the dummy electrodes 7 pick up the same noise. The noise is removed by inputting signals from the cathodes 6A and the dummy electrodes 7 to a differential amplifier such as an IC connected to the photodiode. An Au electrode 9 is mounted on a back surface of the photodiode. The noise immunity is improved by setting the potential of the Au electrode to ground potential.
However, in the prior-art photodiode with improved noise immunity, because the surface of the chip is covered with the two types of opposing Al electrodes, a light-receiving area decreases and signal components weaken if the chip size is the same, which results in a decrease in SN ratio. This decrease has an adverse effect on the improvement of the noise immunity. Furthermore, in the case of the IrDA communication, light is incident to the entire chip and it takes some time for photoelectric current generated in the periphery of the chip to reach the light-receptive section, which results in a delay in response.
The present invention provides a photodiode including a high-concentration first conductivity-type substrate; a low-concentration first conductivity-type epitaxial layer formed on one face of the substrate; a second conductivity-type diffusion layer formed in the epitaxial layer, the second conductivity-type diffusion layer serving as a light-receptive section; a pair of electrodes comprising a cathode and an anode, one of the pair of electrodes being mounted on the second conductivity-type diffusion layer and the other being mounted on another face of the substrate or one of the pair of electrodes being mounted on the second conductivity-type diffusion layer and the other being mounted on the epitaxial layer except the second conductivity-type diffusion layer; and a third electrode mounted in an outer periphery or an inner periphery of a region occupied by the diffusion layer on the epitaxial layer, the third electrode not contacting the pair of electrodes.
In the present invention, the third electrode is electrically insulated from the anode and the cathode. The third electrode is formed proximate the periphery of the photodiode, that is, on the low-concentration first conductivity-type epitaxial layer and in the outer periphery or the inner periphery of the region forming the second conductivity type diffusion layer, preferably in such a manner that the third electrode covers the periphery of the photodiode light-shieldingly.
The third electrode may be formed by the same process as usually used for forming electrodes such as anodes and cathodes in the field of art. In the present invention, for light shielding over the periphery of the photodiode, a metal such as Al, Cu, an alloy of Cu with Al, a laminate of Al and Cu may be used in a specific thickness, for example, 1 to 3 xcexcm in the case of Al.
According to the present invention, a photodiode can be provided which operates at high speed, has good noise immunity and does not mis-operate even in communications across a distance ranging from several tens of centimeters to several meters because the third electrode (a dummy electrode) is disposed proximate the periphery of the photodiode in a light-shielding manner.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.